The present invention relates generally to tape recorders and specifically, to a visual indicator for indicating the amount of headroom available on the tape. "Headroom" is an indication of the amount of additional high-frequency signal that can be recorded on the tape, without the tape becoming saturated and usable to accept any more signal.
Tape recorders generally, and especially tape recorders utilizing compact cassettes in which the tape moves at a relatively slow speed, have suffered from insufficient signal-to-noise ratio and frequency response. In order to overcome these difficulties, a number of noise-reduction systems have been utilized. One commonly used noise-reduction system identified by the trademark "Dolby", pre-emphasizes low-amplitude, high-frequency signals in a predetermined manner before recording on the tape. During playback, the signals are de-emphasized in a predetermined manner opposite to that of the recording pre-emphasis. This pre- and de-emphasis of the signal provides a significant improvement in the signal-to-noise ratio of the recording media.
A further improvement which has recently been added to the Dolby system is known as the Dolby HX headroom extension system. In this system, the control signal usually employed to vary the amplitude of the high-frequency signals is utilized to inversely vary the bias being applied to the tape. When high-amplitude, high-frequency signals are detected, the bias signal is reduced. Simultaneously, the amount of recording pre-emphasis is reduced in order to compensate for the increase in sensitivity to high frequencies caused by the reduction in the recording bias field. The result of changing the bias and pre-emphasis parameters is an improvement in headroom of approximately 10 dB, while maintaining frequency response in accordance with international (IEC) standards.
However, the bias signal is reduced by the Dolby HX system only over a certain range because the bias oscillator current has a set minimum level. Thus, after a certain point, further high-amplitude, high-frequency signals will not cause a further reduction in bias. Thus, after the minimum bias level is reached, tape saturation may again take place. Accordingly, it is desirable that the operator of the tape recorder be aware when the maximum possible headroom extension of the noise reduction circuitry has been reached so that the recording level may be safely kept within the capabilities of the system.
It is accordingly an object of this invention to provide an indicator system for a tape recorder for use with noise reduction circuitry including a headroom extension feature.
It is a further object of this invention to provide an indicator system which signals the possible overload of the tape recorder headroom extension circuitry.
It is another object of this invention to provide an indicator circuit for use with a headroom extension system that indicates various stages in the operation of the headroom extension circuitry.
Still another object of this invention is to provide an indicator for use with tape recorder headroom extension circuitry that will not give confusing signals to the operator.
The invention as disclosed herein, provides an indication of the maximum capabilities of the system and also indicates the various stages in the operation of the headroom extension system to the operator. The indicator of the invention provides a series of three level indications. A first indication is given when the headroom extension circuitry is operating at or below a level that which will cause the bias current to be reduced. A second indication is given when the minimum bias threshold has been reached, to indicate that no further headroom extension can occur. A third indication is given that tape saturation is occurring to indicate that the recording levels should be reduced.
The indicator circuitry is designed so that only one of the indicator lights is lit at any particular time. This feature prevents the operator from becoming confused because of the simultaneous presence of a safety and a warning signal. This design also prevents the headroom extension circuit indicators from appearing similar to any recording level indicators.